1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus having the zoom lens system, which are suitable for, for example, video cameras, electronic still cameras, and TV cameras (cameras for broadcasting).
2. Description of the Related Art
Today, image pickup apparatuses, such as video cameras, digital still cameras, cameras for broadcasting, and cameras using silver-halide film are sophisticated and are reduced in size. Image-pickup optical systems, such as a zoom lens, used in such cameras should be therefore compact in size and highly efficient. In other words, a compact zoom lens system having a small total length with a large zoom ratio and high resolution is required. Furthermore, there is demand for a retractable zoom lens system in which, when image pickup is not carried out (when not used), the lengths of the intervals between the lens units are set smaller than the lengths in an image pickup state. In this manner, the thickness (i.e., the length in the direction of the optical axis) of the entire image pickup apparatus is decreased.
A four-lens-unit zoom lens system, known to satisfy at least some of the above demands, includes four lens units having positive, negative, positive, and positive refractive powers, in this order from the object side to the image side of an imaging apparatus, and carries out zooming by moving each of these lens units. U.S. Pat. No. 7,719,774 (U.S. Pat. No. '774) describes an example of the four-lens-unit zoom lens system. In U.S. Pat. No. '774, the four-lens-unit zoom lens system includes two lens elements (a first lens unit that consists of one negative lens element and two positive lens elements) and a second lens unit that consists of three lens elements (negative, negative, and positive lens elements disposed in this order).
In general, when the number of lens elements in a lens unit constituting a zoom lens system is large, the length along the optical axis of the lens unit increases. Moreover, when the displacement of the lens units is large during zooming and focusing, the total length of the lens system increases. As a result, the retracted length increases, making the lens system less suitable for a retractable zoom lens system. With a zoom lens system used in an image pickup apparatus, it is important that the size of the entire lens system be reduced while achieving a desired zoom ratio and maintaining optimum optical performance in all zoom ranges. To achieve this, it is necessary to appropriately set the movement conditions of the lens units for zooming, the refractive power of the lens units, and the lens configuration of the lens units.
For example, reducing the size of the entire system while maintaining a desired zoom ratio can be achieved by increasing the refractive powers of the lens units in the zoom lens system. However, with such a zoom lens system, the variation in aberration as a result of zooming becomes large, and it becomes difficult to achieve optimum optical performance in all zoom ranges. In the above-described four-lens-unit zoom lens system, to acquire a large zoom ratio and a small-sized lens system while achieving optimum optical performance, it is important to appropriately set the refractive powers of the lens units, the lens configurations, and the movement conditions of the lens units for zooming. In particular, it is important to appropriately set the lens configuration of the first lens unit. Without such appropriate configurations, it is extremely difficult to acquire a zoom lens system suitable for a retractable structure having a large zoom ratio and excellent optical performance in all zoom ranges.